Absolutely rent the Grand Marquis.
Pluses:
They ride as smooth as butter, will have under 20,000 to 30,000 miles on them and some of them are very close to brand new, bulletproof HVAC, safe in a wreck, no maintenance issues to worry about, dead reliable in the first place and if something happens on the road, just get another from Hertz, 26+ mpg on highway, huge amount of luggage space, your personal cars will be safe staying at home, seats 5 in great comfort, build quality is great, all issues gone, (car has been in production in this BOF style essentially unchanged since 1998), nice to look at, people on highway may think you are a state trooper and will get out of your way on the highway, servers that see you come into a roadside restaurant
will automatically prepare you to give you the "senior discount"
.
Minuses:
Will only have a donut spare tire, ( unless the trailer tow package is added, which rental companies never do) mushy handling, road feel is vague and numb, think: balloon floating down a river, this is the last year they will be in production, seats less ergonomically correct than a Mercedes Benz, likewise the instruments and controls. Only available in LX model with leather seats, and fancy wheels, the GS base with cloth seats is gone after the 2008 models.
No longer available with the "de Sade" option.
conclusion:
No brainer, rent the Mercury.
Plus:
They are so good you will probably think about getting a one year old rental return for yourself in a year, as about $15,000 right now will buy you a used '09 rental LX in your favorite color and whatever of the 3 year 36,000 mile warranty is left. Possible service records and history may be available. CARFAX will show where it was placed into rental service when new.
With decent maintenance they could easily last 200,000 - 300,000 miles.
I have owned their cosmetic cousins, Crown Victorias, and dollar per mile, they are the cheapest vehicles to run I have ever owned since 1967 . They can really take hard use, any taxi driver or policeman will assure you, just ask.